The present invention relates to a holding cassette for precision substrates, such as semiconductor single crystal silicon wafers, and a method for the preparation thereof. The method of the invention is an injection molding method for the preparation of a cassette for holding various kinds of precision substrates, such as semiconductor silicon wafers, hard disk substrates for magnetic recording medium, fused silica glass plates of photomasks for use in photolithography and so on, in storage and transportation. In particular, the invention provides a method for the preparation of a cassette for holding precision substrates suitable for mechanical loading and unloading of the substrates by means of an automatic machine.
It is conventional that various kinds of precision substrates such as semiconductor silicon wafers and the like are contained during storage and transportation in a box container, referred to as a wafer carrier hereinafter, as illustrated in FIG. 6 of the accompanying drawings, in order for the substrate materials, referred to simply as wafers hereinafter, to be prevented from contamination by deposition of dust particles and mechanical damage by vibrations and shocks. As is illustrated in FIG. 6, which is a perspective view of a wafer carrier as disassembled into parts, a plurality of wafers W are held on a wafer cassette 10 in a parallel alignment each separated by a narrow gap space from the adjacent wafers and the wafer cassette 10 holding the wafers W is put into the box bottom b of the wafer carrier in a freely demountable fashion. A covering d is mounted on the box bottom b with intervention of an elastic sealing member c such as a rubber gasket fitting the open peripheries of the box bottom b and the covering d to ensure air-tight sealing of the wafer carrier. The wafers W or the wafer cassette 10 is secured in position by means of a wafer presser member e coming into contact with the upper peripheries of the wafers W under elastic resilience. The loading and unloading of the wafers W on and from the wafer cassette 10 are usually conducted in a clean room freed from floating dust particles as completely as possible so as to prevent deposition of microscopic dust particles on the wafers W.
A typical wafer cassette is that for semiconductor silicon wafers of a large diameter as is illustrated in FIGS. 7, 8, 9, 10 and 11 showing a perspective view, a plan view, a front elevational view, a cross sectional view as cut and viewed along the direction indicated by the arrows X--X in FIG. 8 and a partial cross sectional view as cut and viewed along the direction indicated by the arrows XI--XI in FIG. 9, respectively. The wafer cassette 10 is made from a thermoplastic resin such as polypropylene resins (PP), polycarbonate resins (PC), polybutylene terephthalate resins (PBT), polyether-ether ketone resins (PEEK), perfluoroalkoxy fluorocarbon resins (PFA) and the like and integrally molded by the method of injection molding. The main part of the wafer cassette 10 consists of a pair of opposite disks 11, 12 integrally connected together with four connecting bridges 13, 14, 15, 16. Each of the disks 11, 12 is provided with a thick-walled rib 17 along the upper periphery, a thick-walled area 18 along the lower periphery and a pair of vertical ribs 19, 19 each running close to the lateral periphery of the disk 11 or 12 for the purpose of reinforcement. Each of the bridges 13, 14, 15, 16 is provided on the inwardly facing surface with a plurality of vertically running parallel grooves 20 each having a V-shaped cross section to receive the periphery of a silicon wafer W. The upper reinforcement rib 17 of the disk 11 or 12 is provided at each end with a positioning groove 17A having a V-shaped cross section. The lower thick-walled area 18 is also provided at the center with a vertically running positioning groove 18A having a V-shaped cross section.
In the wafer cassette 10 described above, the wafers W are each inserted into a set of the receptacle grooves 20 in the bridges 13, 14, 15, 16 at the periphery to be supported thereon. The wafer cassette 10 thus holding the wafers W is then inserted into the box bottom b of the wafer carrier and air-tightly sealed therein by mounting the covering d with intervention of the elastic sealing member c therebetween to ensure cleanness and safety of the wafers W in storage and transportation. When wafers W are loaded and unloaded on and from the wafer cassette 10 by using a loading-unloading machine, the machine is secured at the proper position by bringing the tip of one of plural positioning pins 21 thereof into contact with the bottom of the V-shaped groove 17A, 18A of the thick-walled parts 17, 18 to serve as a ruling point as is shown in FIG. 11.
The above described conventional wafer cassette 10 has several problems to be solved relating to the reliability of the operation of the loading-unloading machine. Due to the large thickness of the upper reinforcement rib 17 and lower thick-walled area 18 on the side disk 11, for example, these parts unavoidably exhibit a large thermal shrinkage in injection molding so that it is sometimes the case, as is shown in FIG. 12, that the groove surface of each of the positioning grooves 17, 18 has a cross section which is not exactly V-shaped but is curved with concavity so as to disturb the positioning pin 21 of the loading-unloading machine from properly contacting with the positioning grooves 17, 18 and eventually resulting in inoperability of the machine.
As a remedy for the above described problem due to the large thickness of the thick-walled parts 17, 18, a method is proposed in which, as is shown in FIG. 13, a part 22 of a metal mold for injection molding is applied to the disk 11 at the inner surface so as to decrease the wall thickness at the positioning grooves 17A, 18A. Alternatively, as is illustrated in FIG. 14, each of the positioning grooves 17A, 18A is shaped as a separately molded member 23 having an M-shaped cross section which is adhesively bonded to the disk 11.
The above mentioned former method described by making reference to FIG. 13 has another problem that, since the part 22 of the metal mold forms an undercut, the metal mold must be provided with a sliding mechanism with complicacy so that the cost of the metal mold is necessarily increased. In addition, the inwardly facing surface of the disk 11 necessarily has a cavity or recess at the positioning grooves 17A, 18A playing no inherent role in the performance of the disk 11 so that the efficiency of the cleaning treatment of the wafer cassette 10 is decreased so much resulting in eventual deposition of dust particles to cause a problem in the cleanness of the wafer carrier.
The above mentioned latter method described by making reference to FIG. 14 also has problems that, needless to say, an additional metal mold must be prepared for molding of the separate member 23 and the thus separately molded member 23 must be subsequently bonded to the disk 11 so that the costs of the products are increased so much due to the initial investment for the additional metal mold and the labor-cost for the bonding operation.